1. Field of the Invention
The present invention relates to process for producing an aromatic carboxylic acid. More particularly, the present invention relates to a process for producing an aromatic carboxylic acid, particularly 2,6-naphthalenedicarboxylic acid, by the liquid phase oxidation of an aromatic compound substituted with alkyl groups, wherein catalyst components are efficiently recovered from a mother liquor obtained after separation of crystals from the reaction liquid of the liquid phase oxidation.
2. Description of the Related Arts
Aromatic carboxylic acids, particularly aromatic dicarboxylic acids such as terephthalic acid and 2,6-naphthalenedicarboxylic acid, and esters thereof are useful as raw materials for polyesters which are used as fibers and resins.
Aromatic carboxylic acids are produced by liquid phase oxidation of aromatic compounds substituted with alkyl groups in the presence of a heavy metal catalyst. For example, in processes proposed in the specifications of Japanese Patent Publication Showa 34(1959)-2666 and Japanese Patent Publication Showa 56(1981)-3337, an aromatic dicarboxylic acid is obtained by oxidation in a solution containing a lower aliphatic carboxylic acid in the presence of a catalyst containing a heavy metal, such as cobalt and manganese, and bromine.
In the above processes, the reaction product is obtained in the form of a slurry containing solid substances after the oxidation, and the aromatic carboxylic acid is obtained from crystals separated by solid-liquid separation of the slurry. The mother liquor which is obtained as the filtrate after the solid-liquid separation of the slurry contains useful catalyst components, such as expensive heavy metals such as cobalt and manganese and bromine compounds. It is necessary that the catalyst components be recycled in order to use these components efficiently.
The simplest method for the recycling is to reuse the mother liquor without any treatment. However, various organic impurities formed as byproducts in the reaction and inorganic substances formed by elution of materials of apparatuses are present in the mother liquor. These impurities are concentrated during the recycling and occasionally have serious adverse effects on the oxidation.
Therefore, the direct recycling of the mother liquor is limited, and catalyst components which are not reused by the recycling of the mother liquor must be recovered. Various processes have heretofore been proposed to recover catalyst components which are not reused by the recycling of a mother liquor.
For example, water and an alkali metal carbonate are added to a residue obtained after removal of a solvent from a mother liquor, and metal components of a catalyst are precipitated in the form of carbonates, which are then dissolved in acetic acid to recover the metal components (Japanese Patent Application Laid-Open No. Showa 48(1973)-66090). In another example, oxalic acid is added to a mother liquor which is not recycled, and metal components of a catalyst are recovered as oxalates (Japanese Patent Application Laid-Open No. Heisei 2(1990)-203939).
It has been known that metal components can be recovered with anion exchange resins from a mother liquor which is not recycled. For example, cobalt and bromine in the mother liquor is adsorbed with a strong basic anion exchange resin after the ratio of bromine to cobalt has been adjusted in a specific range, and the adsorbed cobalt and bromine are eluted with acetic acid containing water to recover organic substances containing cobalt (Japanese Patent Application Laid-Open No. Showa 53(1978)-133574). In another example, a mother liquor is first brought into contact with an anion exchange resin in the bromide form so that heavy metals are adsorbed with the anion exchange resin, and the adsorbed heavy metals are subsequently recovered from the ion exchange resin by elution. The treated mother liquor is then brought into contact with a weak basic anion exchange resin so that bromine ion is adsorbed with the anion exchange resin, and the adsorbed bromine ion is subsequently recovered by elution from the anion exchange resin (Japanese Patent Application Laid-Open No. Showa 53(1978)-104590).
When metal components are recovered as carbonates or oxalates, auxiliary agents such as alkali metal carbonates and oxalic acid are necessary in amounts exceeding the equivalent amounts, and the processes are not economically advantageous. Moreover, recovery of the carbonates and oxalates requires complicated operations such as neutralization, precipitation and separation of metal salts and excessive labor.
The processes using an anion exchange resin are superior to the above processes in that such agents are not necessary. However, in order to achieve complete adsorption of heavy metals, bromine ion is necessary in an amount by mol about twice or more as much as the amount of the heavy metal at the time of the adsorption. The bromine ion is recovered simultaneously when the heavy metals are recovered. Therefore, the ratio of bromine ion and the heavy metal components in the oxidation is naturally restricted, and the oxidation is conducted in the presence of a large relative amount of bromine ion.
Although it is necessary that the ratio of bromine ion to heavy metals be kept at a specific value or more in the oxidation, bromine ion in an excessively large amount does not show further contribution to the improvement of the reaction, and the amount of bromine discharged to the outside of the system as organic bromine compounds in the off-gas increases. This causes loss of bromine, and the possibility of corrosion also increases. Therefore, it is not preferable that the ratio of bromine ion to heavy metals in a catalyst must be kept at a high value in the liquid phase oxidation.
In the processes using carbonates and oxalates, the metal components are concentrated to form solid substances. In contrast, metals adsorbed with an anion exchange resin are recovered by elution with acetic acid containing a large amount of water. The degree of concentration of the eluted ions is decided by the concentration during the elution, and occasionally the degree of concentration of the eluted ions is insufficient. When the degree of concentration of eluted ions is insufficient, recycling of the recovered catalyst directly into the reaction system increases the concentration of water in the reactor, and this adversely affects the reaction. Therefore, an additional treatment such as removal of water before the recycling is necessary, and this consumes additional energy.